Heat pumps are temperature modification devices which are typically employed to heat an interior space. Heat pumps operate to transport heat from colder exterior air to warm the interior space. This heat transfer is achieved via control of the liquid/gas state change of a refrigerant.
A compressor receives the refrigerant in a gaseous state and through the introduction of pressure changes the state of the refrigerant into a liquid. This process will raise the temperature of the refrigerant. An interior heat exchanger enables heat transport from the hot refrigerant into the air of the interior space. Typically a fan is employed to transport interior air over the interior heat exchanger to facilitate this heat transfer.
The liquid refrigerant is then routed to a evaporator. In the evaporator, the pressure provided by the compressor is released. This causes the refrigerant to vaporize from the liquid state into the gaseous state. Much of the heat of the liquid refrigerant is needed to provide the heat of vaporization. As a consequence, the gaseous refrigerant which emerges from the evaporator is at a much lower temperature than the entering liquid refrigerant.
This lower temperature gaseous refrigerant is then routed to an exterior heat exchanger. This exterior heat exchanger is similar to the interior heat exchanger, except that heat flows from the exterior air into the colder gaseous refrigerant. As in the case of the interior heat exchanger, the exterior heat exchanger typically has an exterior fan to transport exterior air over the exterior heat exchanger to facilitate the heat transfer. The gaseous refrigerant, with its temperature elevated by heat from the exterior air, is then routed to the compressor to repeat the cycle.
The net result of this cycle is the transportation of heat from the colder exterior air to warm the interior air. The temperature of the liquid refrigerant from the compressor would typically be 110 degrees Fahrenheit. The refrigerant would typically be cooled to approximately 100 degrees Fahrenheit in the interior heat exchanger by heating the interior air which would be approximately 70 degrees Fahrenheit. The gaseous refrigerant emerging from the evaporator would typically be much colder, approximately 0 degrees Fahrenheit. Exterior air in the range of 60 degrees Fahrenheit to 35 degrees Fahrenheit would typically heat the gaseous refrigerant to a temperature of approximately 28 degrees Fahrenheit. By thus controlling the liquid/gas state changes of the refrigerant it is possible to transport heat from the colder exterior to heat the warmer interior space. The amount of electrical energy required to transport this heat (the electrical power consumption of the compressor and the interior and exterior fans) is generally less than the electrical energy equivalent of this heat. Thus a heat pump provides greater heating than an electric resistance heater using the same amount of electrical power.
Heat pumps have some disadvantages and limitations which prevent their more widespread use. Firstly, heat transport mechanism is based upon the limited temperature differential achieved by converting the refrigerant from a gas to a liquid and then from a liquid back to a gas. This temperature differential must be greater than the temperature difference between the interior space and the exterior in order for the desired heat transfer to take place. In addition, the heat transport mechanism is most efficient when the temperature difference between the interior and exterior is minimal. Thus the heat transport process is least efficient at the same time the need for heat transfer is greatest, when the exterior ambient temperature is very low. As a consequence a heat pump system is often teamed with an auxiliary heating unit, such as a gas or oil fired furnace, for use when the heat pump is inadequate to provide the desired interior temperature.
Secondly, there is a further factor that reduces the usefulness of heat pumps at low exterior ambient temperatures. The formation of frost on the exterior heat exchanger severely limits the usefulness of heat pumps. Because the refrigerant can have a temperature in the range of 0 degrees Fahrenheit, heat transfer could theoretically take place for exterior ambient temperatures below freezing 32 degrees Fahrenheit). However because of the low temperature of the refrigerant in the exterior heat exchanger, frost tends to form on the exterior heat exchanger from freezing of the humidity in the exterior air even when the exterior ambient temperature is above freezing. Typically frost would begin to form at exterior ambient temperatures in the range of 35 degrees Fahrenheit to 37 degrees Fahrenheit. The build up of such frost tends to insulate the exterior heat exchanger from the exterior air, thus inhibiting the heat transport process.
In accordance with the prior art, there are systems which reverse the connection of the interior and exterior heat exchangers to provide defrosting. This results in the transport of the hot liquid refrigerant to the exterior heat exchanger causing the frost to be melted. Unfortunately, this causes the heat pump to act as an air conditioner, transporting heat from the interior to the exterior, generally at the very time that heating is most desired. Such a defrosting operation also consumes energy which does not contribute to heating. Detection of the proper times to defrost the exterior heat exchanger would thus save energy.
In the prior art there are known systems to detect the build up of frost or the conditions which are known to cause such build up. One technique known in the art involves defrosting based upon the total time of operation of the compressor of the heat pump. Such systems typically employ a bimetal snap switch in the compressor circuit which is heated by the electric current supplied to the compressor. When the duty cycle of operation of the compressor and the time of the current operation reach a limit set by the characteristics of the bimetal snap switch, then the bimetal snap switch trips. This interrupts the compressor and triggers a defrosting operation. Such a system does not take into account the exterior conditions, such as temperature and humidity, which control the likelihood of frost formation. Thus this system can only provide an approximation of the time when defrosting is needed.
Another system known in the prior art employs the difference between the exterior ambient temperature and the exterior heat exchanger temperature to determine when defrosting is required. When this difference exceeds a predetermined amount, based upon the exterior ambient temperature, then a defrosting operation is begun. This technique detects the results of insulation of the exterior heat exchanger from the exterior air due to frost formation and is thus responsive to the particular ambient conditions. Such systems are not ideal for two reasons. These systems require a measure of two temperatures, generally requiring two temperature sensors. In addition, the triggering temperature difference, which is typically formed from a linear function of the exterior ambient temperature, is ordinarily a compromise employed for a number of different heat pumps. Further, it is known that the temperature difference upon frost formation for any particular heat pump will change due to aging caused by deterioration of motor bearings, partial loss of refrigerant and other factors. Thus this defrost operation criteria is only an approximation for any particular heat pump at any particular point in its useful life.
The two factors noted above limit the usefulness of the heat pump in certain climates. If the exterior ambient temperature will be below freezing for any significant portion of the heating season, then either heat pumps are only rarely installed or heat pumps must be backed up with an auxiliary heating unit. This results in the requirement for extra equipment which is only intermittently used. The prior art method for melting frost on the exterior heat exchanger places an additional heating load on the heating system at the same time that heat is most needed by cooling the interior space in order to heat the exterior heat exchanger.
Studies of the temperature patterns of many U.S. cities show that a reduction of only a few degrees in the lowest operating temperature of a heat pump would greatly increase the areas where heat pumps could be used exclusively and greatly reduce the need for auxiliary heat in other regions. Any method Of operation of a heat pumps that can more reliably detect the presence of frost would enable better utilization of powered defrosting and therefore provide such an improvement in the lowest operating temperature. Therefore it would be very useful in the heat pump field to provide a method for reliable frost detection.